Communication within a large process control system typically utilizes a plant loop consisting of a cable and a series of nodes or drops off of the cable permitting transmission of data between control modules and other modules within the system or between control modules and various data acquisition devices.
In prior art plant loop communication systems, a typical form of communication involves sending field I/O points to a data acquisition device for display and archiving purposes. The technique that is used for the foregoing is to input the data from a field I/O point into a control module. That data is then passed across the module bus to a bus interface module which transforms the data into a message which is passed out onto the plant loop. The messaging capability on the plant loop is such that initially each I/O point requires a separate message on the plant loop. This situation has been improved to where several pieces of data can be sent within the same message but each message is still transmitted to only one destination.
The data is typically transmitted as exception reports, i.e., a piece of data is transmitted across the loop only when it has changed significantly or has not been reported for some pre-determined time period. In larger process control systems, the same piece of data must be sent to many different devices each requiring a separate message, thus potentially overloading the loop. Other inherent problems with the prior art plant loop communication system include node recognition, i.e., a node not recognizing when another node is busy. This, in combination with the large number of exception reports that are sent when a node is restarted, causes the destination node to become busy again. The foregoing results in a thrashing condition where a node is seen to be toggling on-line and off-line, depending upon whether it is busy or not. With larger systems having many operator consoles and many I/O points within a single process control unit, this becomes a very serious problem.
A third problem which is significant is the size of the message that can be transmitted on a loop. The maximum size message is 127 bytes and the way that the message is packed, or how much data is packed into the message, is completely determined by the source of the data with no consideration for the impact the message has on the loop. If the loop is particularly busy, a process control unit that has a large amount of data to send still attempts to force the data out onto the loop which aggravates the situation. If the loop is lightly loaded, i.e., there is additional room for messages to be transmitted on the loop, the bus interface module still packs large messages if data is available. This results in an inefficient use of the available bandwidth, or message capability, on the plant loop communication system.
One additional problem that is experienced with the foregoing plant loop communication system is the method by which the various drops on the system receive time information. It is necessary for all drops on the loop to be time synchronized. The method used to achieve time synchronization is to have one node act as a master. Through a series of two messages, an attempt is made to synchronize each node to the node acting as the master. This approach typically results in a time shift of two to three seconds between nodes on a single plant loop. The entire method can take approximately three seconds per drop on the system which results in heavily loaded systems waiting several minutes before all the nodes are synchronized to the same time.